Remove Bluetooth from multiplayer

Multiplayer now runs over Wi-Fi (LAN) only.

- Delete BluetoothTransport and drop it from LobbyController's transport
  list, leaving LanTransport as the sole transport.
- Drop TransportKind.BLUETOOTH and the now-unused RFCOMM_UUID constant.
- Remove the BLUETOOTH/BLUETOOTH_ADMIN/BLUETOOTH_CONNECT permissions and
  the android.hardware.bluetooth feature from the manifest, along with
  the runtime permission request in MainActivity and the
  requestBluetoothPermissions hook on AppEnv.
- Update lobby copy so an empty transport list and an empty host list
  talk about Wi-Fi rather than pairing, and refresh the comments and
  README sections that described two transports.

The transport abstraction itself is kept: everything after the handshake
was already transport independent, so adding another one later needs no
changes above LanTransport.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YEsqYAMJKUJoM7UAavm169

app/build.gradle.kts

@@ -101,7 +101,7 @@ dependencies {
     // Settings persistence for sound / animation speed / update preferences.
     implementation("androidx.datastore:datastore-preferences:1.1.1")
 
-    // Wire format for the LAN and Bluetooth game protocol.
+    // Wire format for the LAN game protocol.
     implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.3")
     implementation("org.jetbrains.kotlinx:kotlinx-coroutines-android:1.8.1")